End your agents' turns with their backs against cover objects. It'll lengthen their life expectancy.
Agents can take cover to hide from guards and Security Cameras. Many objects provide soft cover, allowing agents to hide on adjacent tiles; some decor objects also provide tall cover, blocking line of sight completely. Walls and closed doors are also hard cover.
Cover can also be used to hide knocked-out or lethally-wounded bodies of agents or guards, rebooting or destroyed drones, and suspicious items. Even if a guard notices an agent behind cover, they cannot see the agent, nor can they overwatch or shoot them.
Most decor objects and mainframe devices provide soft cover, allowing agents to hide on adjacent tiles. Tiles which are currently in soft cover from enemy vision are shown as “hidden”, and coloured in yellow; this can often be seen more clearly in tactical view. Soft cover only blocks enemy vision, not the agency's vision.
Cover objects only provide cover on adjacent tiles from some directions. An agent adjacent to a cover object is in soft cover from a guard, Security Camera or Turret's vision if they are further away from the guard, camera or turret than the cover object is. In practice this means that when a guard faces a cover object along a grid axis, tiles behind the object or beside it are in cover from the guard; otherwise tiles on the two sides behind the object are in soft cover.
When an agent is taking soft cover, they are usually (but not always) shown either as crouching, or with their back against the cover object, instead of standing upright. This is purely a graphical detail; in particular, when an agent is in cover from multiple directions, the direction they are facing or the way they are crouched does not matter.
Most tall cover objects also provide soft cover. There are a few decor objects such as the K&O office lamp which do not provide soft cover, despite their appearances. To see whether a particular object provides soft cover, hover the cursor over an adjacent tile; a faint “eye” icon indicates that the tile is in soft cover from that direction.
Doors, guards, traitorous drones and Turrets, and Archive Prism while disguised, are never in soft cover. As such, guards will notice when a door on a “hidden” tile is opened or closed, and they will see when a guard on a “hidden” tile is attacked.
Guards, cameras and turrets always see the adjacent tiles in front of them, even if there is a cover object on the same tile as themselves. For example, a Console never provides cover from a Security Camera on the same tile.
Tall cover objects block line-of-sight completely. Unlike soft cover which only hides certain things from the enemy on adjacent tiles, tall cover blocks vision for both the player and the enemy on all tiles behind it. Tall cover also blocks scans by drones and Enforcers, and the area of effect of Flash Grenades and Flash Packs, which are determined by line-of-sight.